Wong B C, Chiu S K, Chow S A
Department of Biochemistry, University of Hong Kong, Hong Kong.
J Biol Chem. 1998 May 15;273(20):12120-7. doi: 10.1074/jbc.273.20.12120.
Escherichia coli RecA protein pairs homologous DNA molecules to form paranemic joints when there is an absence of a free end in the region of homologous contact. Paranemic joints are a key intermediate in homologous recombination and are important in understanding the mechanism for a search of homology. The efficiency of paranemic joint formation depended on the length of homology and the topological forms of the duplex DNA. The presence of negative superhelicity increased the pairing efficiency and reduced the minimal length of homology required for paranemic joint formation. Negative superhelicity stimulated joint formation by favoring the initial unwinding of duplex DNA that occurred during the homology search and was not essential in the maintenance of the paired structure. Regardless of length of homology, formation of paranemic joints using circular duplex DNA required the presence of more than six negative supercoils. Above six negative turns, an increasing degree of negative superhelicity resulted in a linear increase in the pairing efficiency. These results support a model of two distinct kinds of DNA unwinding occurring in paranemic joint formation: an initial unwinding caused by heterologous contacts during synapsis and a later one during pairing of the homologous molecules.
当同源接触区域没有游离端时，大肠杆菌RecA蛋白会使同源DNA分子配对形成平行双链连接。平行双链连接是同源重组中的关键中间体，对于理解同源性搜索机制很重要。平行双链连接形成的效率取决于同源性的长度和双链DNA的拓扑形式。负超螺旋的存在提高了配对效率，并缩短了形成平行双链连接所需的同源性最小长度。负超螺旋通过促进同源性搜索过程中发生的双链DNA的初始解旋来刺激连接形成，并且对维持配对结构不是必需的。无论同源性长度如何，使用环状双链DNA形成平行双链连接都需要存在超过六个负超螺旋。超过六个负超螺旋圈时，负超螺旋程度的增加导致配对效率呈线性增加。这些结果支持了一种在平行双链连接形成过程中发生两种不同类型DNA解旋的模型：一种是在联会期间由异源接触引起的初始解旋，另一种是在同源分子配对期间的后期解旋。
